1. Field of the Invention
This invention relates generally to electrical machines and more particularly to an electrical generating machine or motor having multiple magnetic paths in a single rotor machine.
2. Description of the Related Art
Electrical power machines, both generators and motors, which convert mechanical energy into electrical energy and visa versa have been built for about a century. These machines have been both direct current (DC) and alternating current (AC) types. The conversion of mechanical energy into electrical energy and visa versa, is accomplished by relative motion between magnetic fields and current carrying conductors.
Conventional electrical machines, both generators and motors provide this relative motion between a conductor assembly and a component assembly producing a magnetic field. This movement is generally rotational about a central axis. The moving or rotating component assembly of the electrical machine is generally called a rotor and the stationary component is generally called a stator. As the rotor is turned in the case of a generator, producing the relative motion described above, magnetic lines of flux pass through the conductors producing an electromotive force in the conductors.
Conventional motors likewise require relative motion between a conductor assembly and a magnetic field in order to function. In general, the rotor being driven mechanically or the stator being driven electrically determines whether the machine operates as a generator or a motor respectively.
In a generator, if the magnetic flux is increased, higher current will be produced in the conductors cut by the passing magnetic field and therefore the output of a generator is increased. Similarly, increasing the rotational speed of the rotor may also increase the output current.
Conversely, in a motor, increased current fed to the rotor and in turn being cut by the magnetic field of the stator will produce a higher mechanical torque output.
The primary difference between a motor and a generating machine involves the application of energy input. In a generator, either DC or AC, the rotor is mechanically driven providing the relative motion between a magnetic field and current carrying conductors. In a motor, a rotating magnetic field is provided by electrically driving the stationary stator. The rotating field then induces mechanical rotation of the rotor thus changing electrical energy into mechanical energy.
The following discussion centers on a description of an electrical generating machine. However, the description is similarly applicable to an electrical motor machine.
A variety of designs of generating machines have been developed and placed in commercial use. Objectives of designers and manufactures of these machines have always been the optimization of the power to weight ratio, minimizing the cost, space required, and speed required, increasing efficiency, increasing reliability, decreasing failure rates, noise, and internal temperature rise, etc.
The rotor in a typical electrical generator comprises a central rotor shaft, a core made of a high magnetic permeability material such as iron mounted on the shaft, and a current carrying coil wound onto the core, and two high permeability structures generally called pole pieces enclosing the core and coil. The rotor core and pole pieces may be integral parts of a pole structure. The pole pieces are shaped to form north and south opposing magnetic poles when electrical current is passed through the coil. At one end of the rotor shaft are positioned slip rings for passing electrical exciter current from the stationary portion of the machine to the coil mounted on the rotor.
The rotor may be constructed utilizing an electrical current carrying coil and pole pieces as described above forming electromagnets or by utilizing suitable permanent magnets positioned on the shaft.
Surrounding the rotor is the stator. The stator is generally cylindrical in shape and comprises ring shaped laminations of high permeability material. These laminations are wound with electrically conductive wire and positioned in a spacial configuration surrounding the rotor in the generator housing so as to generate electromagnetic force (EMF) when there is relative motion between the rotor and stator. This EMF causes the flow of electrical current within the windings of the stator when loaded.
Particularly for small electrical power generating machines and specifically for use in motor vehicles, the claw pole alternator has achieved wide acceptance as the most efficient means for producing electrical power for use in an automobile.
In a claw pole alternator, the pole structure is basically a pair of pole pieces generally made of flat circular metal plates having central bores positioned axially on the rotor shaft on either side of the rotor core and coil. Each pole piece has projecting fingers or claws which are bent over the rotor coil in a direction parallel to the rotor axis. When so bent and positioned, each claw interposes between claws of the opposite pole piece. The pole pieces may be manufactured by metal stamping, forging, machining, or other manufacturing processes.
When current is passed through the coil the claws become electromagnetic poles of opposite polarity. As the rotor is rotated, these poles having opposite polarity a magnetic field which alternates in polarity as seen by the stator winding conductors thus producing alternating current at the output of the alternator. In automotive applications, this alternating current (AC) is generally rectified into DC for use in the automotive electrical systems. The AC may also be used directly.
In order to clearly illustrate the differences between conventional machines and the present invention, a discussion of a conventional machine shown in FIGS. 9 and 10 is believed appropriate at this point. FIG. 9 shows a conventional claw pole alternator in exploded perspective view illustrating the relationship of major components. Rotor assembly 10 is placed within stator and end shield assembly 12. On each end of rotor assembly 10 is positioned a bearing 14, each correspondingly fitting within a bearing race, one carried by stator and end shield assembly 12 and one carried by end shield 16. Rotor assembly 10 is enclosed by stator and end shield assembly 12 and end shield assembly 16 mated and held together by a number of bolts 18.
The shaft portion 20 of rotor assembly 10 extending through stator and end shield assembly 12 is fixed to pulley 22 which is in turn belt driven by a prime mover (not shown). On the opposite end of the rotor assembly, shaft 20 extends through end shield 16. Rectifier assembly 24 is bolted to end shield 16. Brush assembly 26 is in turn bolted to rectifier assembly 24 and rectifier assembly cover 28 is in turn positioned over rectifier assembly 24 and brush assembly 26 and secured to end shield 16.
The assembled internal structure of rotor assembly 10 in the conventional claw pole alternator is shown in FIG. 10. As shown, this machine is a twelve pole synchronous AC generator having a single pole structure rotor. Fixed to rotor shaft 20 is a pole structure comprising a pair of pole pieces 30 and 32. Each pole piece 30 and 32 is identical and comprises disc portion 34 having a centrally disposed bore 36 and six equally spaced apart projections or claws 38 which are bent approximately 90.degree. from the plane of disc portion 34 and extend from the outer portion of disc portion 34 so as to project parallel to the axis of shaft 20 over coil 42.
Pole pieces 30 and 32 are positioned on shaft 20 with claws 38 facing in opposite directions and positioned so that claws 38 on pole piece 30 are interposed between claws 38 of pole piece 32. Between pole pieces 30 and 32 mounted on shaft 20 is rotor core 40 which is comprised of a material having a high permeability such as iron or an iron laminate structure. Rotor core 40 is of cylindrical shape having a centrally disposed bore for mounting on shaft 20. Positioned over rotor core 40 is rotor coil 42. The combination of core 40 and adjacent pole pieces 30 and 32 with claws 38 effectively surround rotor coil 42.
When an electrical current is passed through rotor coil 42, a magnetic flux is set up in pole pieces 30 and 32, effectively making pole pieces 30 and 32 magnet poles having opposite polarity. Thus claws 38 on pole piece 30 will have one magnetic, polarity when claws 38 on pole piece 32 have the opposite magnetic polarity. Thus a rotor having twelve poles on the periphery thereof is created.
Positioned outside of assembly 10 is stator core 44 surrounding rotor assembly 10. Stator windings 46 ar in turn wrapped around stator core 44. In order to provide cooling to these stator and rotor windings, fan 48 is attached to shaft 20 adjacent stator and end shield assembly 12.
In conventional design motors, generators and alternators, the rotor assembly includes a single pole structure comprising a set of two pole pieces, a coil and a core. The core may be an integral part of the pole pieces. The geometry of the pole structure depends upon the total flux it has to carry, the maximum rotational speed it must withstand, the method of manufacturing, noise characteristics, etc.
For a given unit size, then, optimizing all parameters in a conventional electrical generating machine or motor machine will allow the production of a certain fixed capacity. Thus if the electrical circuit needs or mechanical torque requirements are greater than this capacity, then either a larger electrical generating or motor unit is needed or an additional unit is needed in order to supply the requirements of the load. Particularly for automotive applications, where space is at a premium, these alternatives are undesirable.
The designs which have been produced for many decades have had limitations and problems of which the following are exemplary. First, present alternator designs have generally low efficiencies. Particularly where increased alternator outputs and smaller engine size are important, efficiency has become extremely important. Second, in present alternator designs, in order to achieve a higher output, longer projecting claws are required. In addition, higher speeds are required which produce excessive noise which is difficult to prevent. Third, relatively thick pole pieces and long claws are required to carry the necessary magnetic flux. Finally, increase in the output of present machines causes winding temperatures to also rise, posing an additional thermal limitation on the design.